For example:
I currently have a stock of 9mm 125gr RN FMJ bullets.
The book I usually rely on, Lyman's #49 lists a 9mm 124gr JHP. Probably a good match... IF I can figure out a reasonably accurate OAL. Unclenick's information on calculating OAL/seating depth is exactly what I needed to know, and I can calculate the OAL for my round nose bullets, IF I know the actual length of the JHP bullets used in the load data.

JBM ballistics lists at least a couple of 9mm 124gr JHP bullets, depending on what brand you're talking about. I have no idea what bullet the Lyman data is based on. The best I can do is make a closest guess, and err on the conservative side. In doing this, it seems I'm always winding up with starting loads that are waaaay too light. I'm sure that this is in part caused by conservative seating depth. I'm just trying to eliminate some of the guesswork.